Compacting of particulate materials



1965 A. R. BOBROWSKY 3,230,236

COMPACTING OF PARTICULATE MATERIALS Filed Jan. 25, 1961 2 Sheets-Sheet 1.

INVEN ALFRED R. BOBR SKY ATTORNEYS United States Patent Ofifice 3,230,286 Patented Jan. 18, 1966 3,230,286 COMPACTING F PARTICULATE MATERIALS Alfred R. Bobrowsky, Florham Park, N.J., assignor to Engelhard Industries, Inc., Newark, N.J., a corporation of Delaware Filed Jan. 23, 1961, Ser. No. 83,989 1 Claim. (Cl. 264120) This invention relates to the compacting of particulate materials and more especially to a method for the high pressure compacting of powdered and/or fibrous materials, for instance powdered metals.

Specimens of solid materials made from powders and/ or fibers are usually fabricated by any of the following processes: pressing and sintering, hot-pressing, pressing and infiltrating by molten materials, flame spraying, slip casting and firing. However, the resulting specimen usually possesses a density less than that of constituent powders and/or fibers because of voids between the grains and particles.

Use of dies for compacting these porous specimens is not entirely satisfactory for the reason that the pressure is not equal in all directions and hence uniform densities are not obtained. While use of hydrostatic pressure results in application of equal pressure in all directions, when compacting these porous specimens using direct hydrostatic pressure, i.e., with the hydrostatic liquid in contact with the specimen, liquid enters the pores and not only produces no compacting but may shatter the specimen on release of pressure. Elastomeric containers have been used around the specimens for compacting the same, and, while such containers are satisfactory at lower hydrostatic pressures, they are not satisfactory for use at high pressures.

The present invention is concerned with a method for compacting particulate materials such as powdered and/ or fibrous materials under pressures greater than usual and without infiltration of the compact by the hydrostatic fluid. In accordance with the method, a mass of discrete particles of the material to be compacted is first compressed and shaped in a preliminary ope-ration, for instance with dies, to preferably generate the desired shape of the final product. The shaped porous low-density coherent mass is then encased in a jacket of high compressibility and preferably of a material which undergoes an abrupt densification or reduction in volume at a. predetermined pressure. A high hydrostatic pressure is then applied to the jacket to further compact the porous mass and materially increase its density. The jacket may then be removed from the product compact, for instance by melting. The product is of high density closely comparable to that of the same specimen material made in a nonporous manner, i.e., without starting from powder or fibers In a modification of the method, the preliminary compressing and shaping operation is omitted, and the powdered and/ or fibrous material is encased as such in the jacket and the method carried out as previously described. The porous mass is encased in the jacket, for example, by cutting an opening in the top of the jacket wall, pouring the discrete particles in, and then plugging and sealing the opening shut with a plug and sealant of the same material as the jacket. Application of the high hydrostatic pressure to the jacket will compact the porous mass of powder to form a coherent mass of high density. This coherent mass can then be formed into the particular shape desired, for instance by stamping, i.e. cutting by pressing with conventional stamping apparatus, cutting alone, etc. While this embodiment of the invention gives good results, the first-mentioned embodiment including the preliminary compressing and shaping operation is preferred for the reason that the desired shape of final product may be attained.

The compressibility of the jacket, which is of a nonelastomeric material, is preferably higher than that of the porous mass from the preliminary compacting and is essentially higher than that of the final compacted product of this invention. Further, the jacket should be of a material that does not harm the porous mass on application or removal. If the jacket is applied to the porous mass in a molten state, it should not appreciably infiltrate the mass as may be controlled if necessary by addition to the mass of suitable non-wetting agents, e.g. lubricating oil.

Materials which undergo an abrupt densification or reduction in volume at a predetermined elevated pressure, and which may be used for the jacket of the present invention are, for example, cerium, cesium, bismuth and low-melting bismuth-containing alloys. Many other materials are known which also undergo such densification or reductions in volume. In general, preferred materials are those in Which such abrupt densification occurs at pressures within convenient operating range for the hydrostatic press. In some cases this densification is a result of a polymorphic transformation from one crystal structure to another closer-packed crystal structure. In other cases it is believed this abrupt densification is due to the shifting of an outer electron to an electron shell which is closer to the atomic nucleus; and in still other cases, abrupt densification occurs under circumstances which are not well understood. In Table I below, the pressure levels at which abrupt densification takes place for three elements are set forth:

Table 1 Element: Pressure-atmospheres Cerium 7,600 Bismuth 25,000 Cesium 44,000

The changes in volume are relatively large with that for bismuth being about 9 percent, and for cesium about 11 percent. The jacket need not be in the form of a pure chemical element. Preferably, the material of the jacket is a low-melting bismuth-containing alloy for instance an alloy of bismuth, lead and tin melting at 203 F. By reason of the jacket undergoing an abrupt densification under high pressure, the encased porous specimen is subjected to an intensified pressure materially higher than that applied to the jacket.

The porous compact of powdered and/ or fibrous material resulting from the preliminary compressing can be encased within the jacket by:

(a) Casting a jacket around the specimen,

(b) Placing two halves of a pre-cast jacket around the specimen and welding tight the seams of the jacket;

(c) Introducing the specimen through an orifice or hole in a pre-cast jacket, and then sealing shut the hole.

Preferred hydrostatic pressures of this invention are in excess of 20,000 atmospheres, more preferably from about 20,00060,000 atmospheres. Pressures applied during the preliminary compressing and shaping are those sufficient to compact the powdered material to the degree necessary to form a coherent mass. The time of the hydrostatic pressure application may range from about one second to about one hour or more.

A binder may be admixed with the powder prior to the preliminary compressing and shaping of the preferred embodiment. Examples of such binder materials are copper for tungsten fibers, and cobalt for bonding carbides such as titanium carbide.

A porous shaped coherent specimen, which has been compressed and shaped from a mass of powdered ferrous metal in a preliminary operation with a die and which is encased in a jacketing material of this invention, but

compacting, is shown in FIG. 1 of the accompanying drawings, FIG. 1 being a longitudinal section through the jacket. In FIG. 1, bolt having head 11 and threaded end portion 12 is encased in jacket 13 of a low melting bismuth-containing alloy. FIG. 2 is a section taken on line 22 of FIG. 1. FIGURE 3 is an elevational view partially in section showing the apparatus for application of hydrostatic pressure in accordance with this invention.

In applying the hydrostatic pressure, the jacketed specimen to be compacted is placed within the pressure cylinder or chamber of a conventional and well known hydrostatic pressure apparatus Qnesuch apparatus comprises a hydraulic arrangement disclosed in my copending patent application Serial No. 819,942, now abandoned, filed June 12, 1959. The upper pressure limit of this apparatus might be lower than is needed for the densification desired; and the use of the preferred jacket of this invention, which has a high compressibility and undergoes an abrupt densification at a predetermined high pressure, achieves higher pressures than can be attained by use of the conventional apparatus. Referring to FIGURE 3, which shows the hydrostatic pressure-application apparatus of application Serial No. 819,942, an article 5 to be further compacted and encased in jacket 6 of a lowmelting bismuth-containing alloy is surrounded by hydrostatic liquid in pressure chamber 7 of the apparatus. The low-melting alloy of jacket 6 may be an alloy of bismuth, lead and tin melting at 203 F. Pressure chamber 7 is formed by a cylindrical bore through thick-walled steel tube 8, the outer portion of tube 8 being tapered to fit an internal taper in a stack of restraining rings 9, 10, 11 and 12 which surround tube 8. Such restraining rings are of thick tool steel, and the stack of such restraining rings bears with its upper surface 13 against heavy steel ring or annular member 14. Annular member 14, having central opening 15 through which the smaller diameter end-portion of tube 8 extends, is maintained in its position by means of bolts 16 and nuts 17. The bolts 16 together with the base 18 and an upper piston enclosure 19 form the chassis of the apparatus. Oil is pumped under pressure through conduit 21 into enclosure 22 having piston 23 therein, whereby piston 23 is urged upwardly to force tube 8 into the surrounding taper of the restraining rings.

Oil is then pumped through conduit into an upper portion of piston enclosure 19, whereby the piston mounted in enclosure 19 is urged downwardly to drive both rod 24 and piston 25 associated therewith in a downward direction. Piston 25 is secured to rod 24 by axially protruding pin 26 secured to piston 25 and engaging axial recess 27 in rod 24, with low melting point alloy gasket 28 arranged between rod 24 and piston 25. As a result of the downward movement of rod 24 and piston 25, a high hydrostatic pressure greater than the predetermined pressure hereafter mentioned is hydrostatically applied to the jacketed article within chamber 7. This hydrostatic pressure is distributed evenly to the jacket 6 and hence to the encased article 5 through the liquid in chamber 7, which jacket material is of a nature such that it undergoes an abrupt reduction in volume at the predetermined pressure. During such hydrostatic pressure application, the assembly of pistons 30 and 31, which are secured together by pin 32 in piston 30 engaging recess 33 in piston 31, with low melting point alloy gasket 34 between pistons 30 and 31, seals the lower end of pressure chamber 7. The assembly of pistons 30 and 31 rests on closing insert 35.

After completion of the pressure treatment, the apparatus is disassembled, the jacketed article removed from chamber 7 and the jacketing material removed from the article.

After completion of the hydrostatic pressure compacting, the jacket can be removed from the com act of 4 higher density by melting. Alternatively, other means of effecting its removal such as, for instance cutting, dissolving, evaporating, cooling until brittle and shattering, and abrading can be employed.

Particulate materials which can be compacted in accordance with the present invention include both powdered and/or fibrous metals and non-metals. Metals include the ferrous metals, high-temperature alloys, for instance high-temperature nickeland chromium-containing alloys, e.g. a 78.75 percent nickell4 percent chromium-7 percent iron.2 percent copper-.05 percent carbon alloy, or a cobalt-chromium-nickel-molybdenum alloy, e.g. a 51.6 percent cobalt--26 percent chromium-lS percent nickel-+6 percent. molybdenum? 1 percent ir-on--.4 percent carbon alloy; and bearing metals, for example bronzes and graphite-containing bronzes. Examples of powdered and/or fibrous nonmetals include carbides, for instance titanium carbide bonded with cobalt, self-bonded tungsten-titanium carbide (no added binder), silicon carbide, etc. Other powdered and/or fibrous non-metallic materials which can be compacted in accordance with the invention include oxides, for instance aluminum oxide, lanthanum oxide, beryllium oxide and magnesium oxide, chalcogenides of lanthanons, i.e. the sulfides, tellurides or selenides of lanthanons, bismuth telluride, lead telluride, lead selenide and gallium arsenide, silicides, for instance MoSi borides, for instance chromium boride, and nitrides, for instance boron nitride. These materials, when in powder form at the outset of the method, i.e., in the form of finely divided free-flowing discrete particles", have a typical particle size of about 15 to microns. When in fiber form, they have diameters of 0.01 inch and below down to whisker size. Additional metallic fibers which can be compacted include metals in the form of whiskers, for instance platinum whiskers dispersed in copper. Such material is prepared by melting the copper, introducing the platinum whiskers into the melt, and cooling to solidify the copper. The term whiskers" is known in this art and is referred to in the article entitled Metals Reinforced With Fibers in Metal Progress, pages 118-121, September 1960 and in Materials in Design Engineering, page 134, September 1960. Additional fibrous non-metals which can be compacted in accordance with the invention include ceramic fibers preferably of high tensile strength, for instance sapphire (aluminum oxide) fibers dispersed in a cobalt-chromium alloy. This material is prepared in manner similar to that for preparing the platinum whiskers in copper.

Application of high hydrostatic pressure to crystalline material of this invention, for instance to metallic crystalline material, e.g. ferrous metals, produces significant and permanent changes in the properties of these materials. For example, the creep resistance and hardness of the iron was permanently greatly increased.

It was concluded that the effect of the high hydrostatic pressure was to increase the number of dislocations in the structure of the material and alter their location and arrangement. Dislocations are defects in crystalline material and are described in detail in the book entitled Dislocations and Plastic Flow in Crystals by A. H. Cottrell, Oxford, Clarendon Press, 1953. The number and form of such dislocations, according to theory, affect certain properties of materials such as hardness, creep resistance, and electrical conductivity but do not affect certain other properties, such as, for example, modulus of elasticity to any great extent.

In accordance with a specific embodiment, a powdered metal such as powdered iron is subjected to a compress ing and shaping by the use of dies- A jacket of a bismuth-- lead-tin alloy melting at 203 F. is then cast about theporous mass. The assembly is then placed in the pressure chamber of a conventional hydrostatic pressureapplying apparatus. Ultra high hydrostatic pressure in excess of 15,000 atmospheres is then applied, to thej aeket and, as a result, the jacket undergoes both a continuous and an abrupt densification or reduction in volume. The result is a great permanent compaction of the porous mass of iron to cause appreciable cold welding of the grains of the porous mass to one another.

The jacket is then removed from the compacted iron by melting. The resulting compacted product is of very high density, and may be closely comparable or superior to that of the same specimen material made in a nonporous manner.

Products which can be produced by the present inven tion include, for example, Wear pins, turbine blades, machine bolts and components of electron tubes.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

A method for compacting a particulate material, which comprises subjecting the particulate material selected from the group consisting of material powder particles and material fibers to a compressing and shaping operation with a die to form a porous shaped coherent mass of shape desired in the final product, encasing the shaped porous mass in a jacket of a non-elastomeric solid material selected from the group consisting of cerium, cesium, bismuth and low melting bismuth alloys of higher compressibility than that of the shaped mass and which jacket material undergoes an abrupt reduction in volume at a predetermined pressure, and applying a high hydrostatic pressure at least as great as the predetermined pressure to References Cited by the Examiner UNITED STATES PATENTS 2,648,125 8/ 1953 McKenna et al. 2,847,708 8/1958 Hamjian et al. 2,941,245 6/ 1960 Cheney. 2,995,776 8/1961 Giardini et al. 3,030,661 4/1962 Strong. 3,030,662 4/1962 Strong. 3,044,113 7/ 1962 Gerard et al.

OTHER REFERENCES Review of Modern Physics, Recent Work in the Field of High Pressures, volume 18, No. 1, January 1946, pages 32-37.

The Resistance of 72 Elements, Alloys and Compounds to 100,000 Kg/CM Bridgman American Academy of Science, vol. 82, No. 2, 1952, pages 169-188.

ROBERT F. WHITE, Primary Examiner.

WILLIAM J. STEPHENSON, MORRIS LIEBMAN,

LESLIE H. GASTON, ALEXANDER H. BROD- MERKEL, Examiners. 

